x86 VISWS: build fix

the 'reboot_force' flag is a notion that non-PC subarchitectures do
not have.

also, unify the X86_BIOS_REBOOT option between 32-bit and 64-bit
and get rid of a few unnecessary Kconfig and Makefile complications
that way.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>